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Old 18th Feb 2023, 8:23 am   #41
peter_sol
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Default Re: PYE P75 radio conversion to guitar amplifier

A lot of the gain will be lost in that tone stack
I look forward to the results, when it's all finished.
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Old 18th Feb 2023, 2:55 pm   #42
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Default Re: PYE P75 radio conversion to guitar amplifier

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Originally Posted by RogerLLL View Post
Attachment 273421

Hi again, I've put the attached schematic together, combination of extracts from various circuits, but it should be easy enough to interpret. Wasn't sure about whether C18, C19 & R9 were still needed, so any advice would be greatly appreciated. Also whether I've made any obvious errors with the first stage layout & component values.
What is your idea behind R9 and C18, connected in series between the wiper of the treble control and ground? I don't see any positive purpose for them. I would think that all they will do is to lead part of the signal to ground (the amount depending on the value of R9), especially if C18 is indeed an electrolytic capacitor, which have relatively high capacitance. I would skip them if I were you.
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Old 18th Feb 2023, 11:47 pm   #43
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Default Re: PYE P75 radio conversion to guitar amplifier

There is also no need for C19. The output transformer uses the integrated choke technique for some extra smoothing to screen and preamp stages.

What was the reason for dropping the plate supply for V3 - it seems like you are going to pull that down to circa 100V across C16 (depending on your aim).

If the input vol pot becomes a main source of hum due to its sensitive signal location and possibly long screened wiring run, then you could move the pot to say R16.

You could put in say a 10k stopper on V2 input, if you aim on having a variety of input signal sources.

The 1Mohm at the input to the tone stack is mainly functioning to avoid the need for any DC voltage rating on the tone stack caps - was that the intent?

Last edited by trobbins; 18th Feb 2023 at 11:53 pm.
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Old 19th Feb 2023, 12:19 pm   #44
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Default Re: PYE P75 radio conversion to guitar amplifier

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Originally Posted by trobbins View Post
There is also no need for C19.
No C19 would mean no signal feed to V3.

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Old 19th Feb 2023, 12:31 pm   #45
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Default Re: PYE P75 radio conversion to guitar amplifier

I'm sure that trobbins meant that you could change C19 into a wire.
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Old 19th Feb 2023, 12:36 pm   #46
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Default Re: PYE P75 radio conversion to guitar amplifier

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I'm sure that trobbins meant that you could change C19 into a wire.
Let's hope so....

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Old 20th Feb 2023, 11:14 am   #47
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Default Re: PYE P75 radio conversion to guitar amplifier

Quote:
Originally Posted by Robert Gribnau View Post
Quote:
Originally Posted by RogerLLL View Post
Attachment 273421

Hi again, I've put the attached schematic together, combination of extracts from various circuits, but it should be easy enough to interpret. Wasn't sure about whether C18, C19 & R9 were still needed, so any advice would be greatly appreciated. Also whether I've made any obvious errors with the first stage layout & component values.
What is your idea behind R9 and C18, connected in series between the wiper of the treble control and ground? I don't see any positive purpose for them. I would think that all they will do is to lead part of the signal to ground (the amount depending on the value of R9), especially if C18 is indeed an electrolytic capacitor, which have relatively high capacitance. I would skip them if I were you.
I actually spotted a mistake in my representation of the existing circuit, so have corrected it as in the attached. However I agree that R9 is not neecessarily achieving anything. It's described as a "signal diode load", though as this is now purely an audio amplifier, I can't really see a reason why this would need to be there. I did try disconnecting it and it didn't really seem to make any noticeable difference, possibly a slight increase in volume with it out of circuit, so I'm inclined to leave it out, especially as I need as much available gain as possible due to the tone stack.Click image for larger version

Name:	Pye_P75_Page_1 no radio + tone stack.jpg
Views:	34
Size:	71.0 KB
ID:	273599
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Old 20th Feb 2023, 11:49 am   #48
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Default Re: PYE P75 radio conversion to guitar amplifier

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Originally Posted by trobbins View Post
There is also no need for C19. The output transformer uses the integrated choke technique for some extra smoothing to screen and preamp stages.

What was the reason for dropping the plate supply for V3 - it seems like you are going to pull that down to circa 100V across C16 (depending on your aim).

If the input vol pot becomes a main source of hum due to its sensitive signal location and possibly long screened wiring run, then you could move the pot to say R16.

You could put in say a 10k stopper on V2 input, if you aim on having a variety of input signal sources.

The 1Mohm at the input to the tone stack is mainly functioning to avoid the need for any DC voltage rating on the tone stack caps - was that the intent?
C19 was again part of the original circuit, and is described as a coupling capacitor, which makes sense, so I left it in by default. However, I've also included a 100nF coupling capacitor which I can see now I didn't need, though I imagine the coupling capacitor should sit before the tone stack, so thinking it probably makes sense to place the tone stack after C19.

The plate supply for V3 is as per the original circuit, though I've noticed the spec sheet lists a plate voltage of just 24v for this tube! That is really low so I'm thinking I probably ought to adjust the values of R11 & R12, or maybe just eliminate R12. There's a feed between those two resistors for the screen of V2, which is listed as 47v which also seems low given that the nominal screen voltage for this tube is 100v. So a lower value for R11 makes sense.

The input volume I'd decided to remove anyway as there's not much point in it being there, as really that's the equivalent of the guitar volume control. Ideally I'd like a gain control and a master volume, so maybe the gain should be in place of the 1 M ohm resistor after V2, leaving the master volume as the existing R10, or would this be better suited to being moved in place of R16?

TBH the 1 M ohm resistor was included purely because I was looking at typical circuits without really understanding the use of such a resistor. I get the need for a coupling capacitor to remove DC current, but not sure about the need for that resistor, so any advice would be appreciated. I'm still learning! Like I say, would this be a good place for a gain control?
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Old 20th Feb 2023, 12:37 pm   #49
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Default Re: PYE P75 radio conversion to guitar amplifier

The typical voltages for the V3 (EBC41) are way higher than those used in this circuit, so there's a good case for getting these higher.
http://www.r-type.org/pdfs/ebc41.pdf
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Old 20th Feb 2023, 1:54 pm   #50
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Default Re: PYE P75 radio conversion to guitar amplifier

Here's an updated schematic with suggested changes highlighted. I've also added all component values which I think should be helpful! I added the 25k resistor in line to the screen of V2 which needs to be 100v nominal, not sure if this resistance value is correct, or how to calculate this. The plate voltages of V2 & V3 should both end up as 194v which fits with the specs for these tubes. I do wonder if the master volume pot should really be after V3 rather than before?
Click image for larger version

Name:	Pye_P75_Page_1 no radio + tone stack v2.jpg
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Size:	74.3 KB
ID:	273618

Last edited by RogerLLL; 20th Feb 2023 at 2:00 pm. Reason: addition
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Old 20th Feb 2023, 5:31 pm   #51
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Default Re: PYE P75 radio conversion to guitar amplifier

If your red crosses through several components mean that you plan to skip them altogether (and instead use pieces of wire there) than your amplifier will do nothing at all. For valves to amplify, they need a load (without any load, there can't be voltage swing). Skipping the anode resistors of V2 and V3 comes down to skipping their loads, resulting in no voltage swing at the anodes. The result is no voltage swing at all, so absolutely no signal will get through (the dead end would already start at the anode of V2).
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Old 20th Feb 2023, 6:05 pm   #52
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Default Re: PYE P75 radio conversion to guitar amplifier

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Originally Posted by Robert Gribnau View Post
If your red crosses through several components mean that you plan to skip them altogether (and instead use pieces of wire there) than your amplifier will do nothing at all. For valves to amplify, they need a load (without any load, there can't be voltage swing). Skipping the anode resistors of V2 and V3 comes down to skipping their loads, resulting in no voltage swing at the anodes. The result is no voltage swing at all, so absolutely no signal will get through (the dead end would already start at the anode of V2).
Ahhh, ok, I get that. I'm trying to keep the plate voltages high, but obviously missing the need for anode load. What values would you suggest? I've attached the voltage table for the Pye P75.
Click image for larger version

Name:	Tube voltages for Pye P75.jpg
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Size:	31.3 KB
ID:	273637
The 24v plate voltage for V3 seems ridiculously low when its rated for a typical voltage of 250v as in the link in my earlier post for the EBC41.
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Old 20th Feb 2023, 7:28 pm   #53
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Default Re: PYE P75 radio conversion to guitar amplifier

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Originally Posted by RogerLLL View Post
I'm trying to keep the plate voltages high...
The 24v plate voltage for V3 seems ridiculously low when its rated for a typical voltage of 250v as in the link in my earlier post for the EBC41.
The application doesn’t need a high anode voltage so using a large value resistor translates to good audio gain. Why do you want high voltages on signal stages?
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Old 20th Feb 2023, 7:42 pm   #54
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Default Re: PYE P75 radio conversion to guitar amplifier

Earlier you posted this link to a Philips datasheet for the EBC41: http://www.r-type.org/pdfs/ebc41.pdf

The numbers in the table on page 61 are valid for a supply voltage of 250 V. But the supply voltage in the Pye P75 is around 194 V. For that you would have to skip R12 = 47K like you planned. But skipping R12 would result in more ripple because you would skip the smoothing filter R12/C28. The remedy for that is to use much more capacitance for C30 than the 16 uF in the Pye. You could try something like 100 uF or even higher (just use 220 uF).

So let's say that you have a supply voltage of 194 V to feed the anode resistor (220K) of V3. From this point you also have to feed the screen grid of V2. That screen has to sit at 42 V while consuming 1.2 mA. So the resistor in between has to have a value of (194 - 42) / 0.0012 = 126666.7 Ohm. A practical value would than be 120K.

Your supply voltage is 194 V instead of the 250 V in the tabel on page 61 of the datasheet, so the cathode resistor has to have a little higher value than in the table. In the table it is 1K8. So I would choose 2K2.

But your remark that the typical anode voltage for the EBC41 is 250 V is not really valid. It's the voltage while measuring at the EBC41 under static conditions (so without any load, so without any voltage swing, and so without any amplification). The voltage drop over the anode resistor of 220K like in the table, is V = I x R = 0,0007 x 220K = 154 V. So the anode voltage would be 250 - 154 = 96 V, so much lower than the 250 V under static conditions.
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Last edited by Robert Gribnau; 20th Feb 2023 at 8:06 pm. Reason: Addition & corrected some typo's
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Old 20th Feb 2023, 8:02 pm   #55
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Default Re: PYE P75 radio conversion to guitar amplifier

You can also get a reasonable idea as to what's what with any load resistance by drawing a load line on the Ia-Va curves in the valve data.

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Old 20th Feb 2023, 9:54 pm   #56
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Default Re: PYE P75 radio conversion to guitar amplifier

Thank you so much for patiently explaining all that! Really useful information, and I feel much more the wiser now. Steep learning curve! I'll revisit the schematic.
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Old 20th Feb 2023, 10:47 pm   #57
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Default Re: PYE P75 radio conversion to guitar amplifier

Thank you.

To (hopefully) add to your understanding of how valves do their amplifying: In the RF stages in a radio, so before detection (by means of diodes) takes places, the dc-voltages at the anodes of those valves are practically the same as their supply voltage because the dc-resistance of the coils (their loads!) are very low. But the impedances for alternating currents of these coils at the frequencies they are designed to operate, are high. So they achieve high voltage swings, so high amplification, at those frequencies.

The power stage of an audio frequency amplifier with an output transformer is very much comparable to that. The dc-resistance of the primary of a (good) output transformer is low. So the anode voltage of the power valve is just a little bit less than the supply voltage. But the impedance for alternating currents of a good output transformer is high (depending on their value for L, being Henry). This results in them being able to swing the anode voltage between close to 2 x the supply voltage and close to 0 Volts.

Note: This effect doesn't apply to pure resistive loads, like anode resistors (that is why the anode voltage with pure anode resistors has to be much lower than the supply voltage, else swinging up and down in voltage would not be possible).

This fact was a real breakthrough in my understanding of this hobby. I hope it will help you too.
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Old 21st Feb 2023, 12:19 am   #58
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Default Re: PYE P75 radio conversion to guitar amplifier

It does, and makes perfect sense, I get how this is fundamental to how tubes are able to amplify at all. So those load resistors are the only way this can happen, by allowing that voltage swing. Again, much appreciated!
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Old 21st Feb 2023, 12:07 pm   #59
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Default Re: PYE P75 radio conversion to guitar amplifier

Here's a good primer, it also gives a worked example of what to do:

https://www.valvewizard.co.uk/Common_Gain_Stage.pdf

Link to the sites home page for small signal and power pentodes etc.:

https://www.valvewizard.co.uk/index.html

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Old 22nd Feb 2023, 11:42 am   #60
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Default Re: PYE P75 radio conversion to guitar amplifier

Thank you! I've also found Uncle Doug's tutorials on YouTube really informative.
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